The present invention relates to micro actuators necessary for information and precision instruments, machine tools, and FA (Factory Automation) and other fields or various production processes of semiconductors, LCD's, display devices, surface mounting, and the like.
The machining accuracy in machining processes is improving from the micron order to the submicron order. Although the submicron processing is usual in the fields of semiconductor and electronic components, there is also a growing demand for ultraprecision machining in the field of machining that is developing together with mechatronics. One of the reasons for enabling the ultraprecision machining is the introduction of a high-precision measurement technology utilizing laser, which can easily measure the displacement on the nanometer order.
Furthermore, electromagnetostrictive elements represented by a Giant-magnetostrictive element and a piezoelectric element have been recently used as a micro actuator together with the introduction of the high-precision measurement technology.
With the discovery of giant magnetostriction at a very low temperature in 1963 as a start, the Giant-magnetostrictive material reputed to be invented chiefly by A. E. Clark in 1972 has recently been promptly put into practical use because the material can obtain a giant magnetostriction two orders of magnitude greater than the conventional magnetostrictive material even at normal temperature.
A typical material has a composition including a rare earth element R and iron Fe at an atom ratio of 1:2, and these include, for example, TbFe2, DyFe2, SmFe2, HoFe2, and ErFe2.
In contrast to the fact that the conventional magnetostrictive material has a magnetostriction of several tens of parts per million, the Giant-magnetostrictive material can obtain 1500 to 2000 PPM (magnetostriction: PPM=ΔL/L×106).
FIG. 12 shows the representative structure of a direct-acting type actuator that employs this Giant-magnetostrictive bulk material, including a Giant-magnetostrictive rod 700, a magnetic field coil 701, a bias permanent magnet 702, a yoke member 704, an output shaft 705, and a bias spring 706.
The bias permanent magnet 702 is arranged so as to boost the operating point of the magnetic field by preliminarily applying a magnetic field to the rod 700 and widen the range of linearity of Giant-magnetostriction with respect to the intensity of the magnetic field. The bias spring 706 is provided for making the slope of magnetostriction steep with respect to the magnetic field by preliminarily applying an appropriate preload (compressive stress) to the rod 700.
The Giant-magnetostrictive actuator has the features of (1) large displacement magnitude, (2) low-voltage drive, (3) high-speed response, (4) large generated load, and (5) non-contact drive capability.
The laminate type piezoelectric element, which has had a comparatively long history and is compared with the Giant-magnetostrictive element, is widely used as a micro actuator for a precision positioning apparatus and so on. However, the aforementioned features (2) and (5) are features of the Giant-magnetostrictive element, but are not owned by the piezoelectric element.
The commodities of OA (Office Automation) and AV (Audio Visual) uses, information equipment, and so on have recently been developed to have decreasing weights and sizes, and micro and fine production technologies have been required for the manufacturing of the commodities. For example, accurate positioning, accurate linear conveyance, a high-speed feeder of small components, very minute flow rate control, and so on are required. The key to the development of these technologies is a high-performance micro actuator.
Actuators are employed not only for uniaxial operation but also as a mechanical unit capable of operating in a complex manner with a high degree of freedom and are also desired to have high accuracy and high response.
The electromagnetostrictive elements such as the Giant-magnetostrictive element and the piezoelectric element can be regarded as idealistic actuators since they have the aforementioned features if their uniaxial characteristics are evaluated.
However, if an apparatus is constructed by combining a plurality of electromagnetostrictive actuators in order to achieve the operation with a high degree of freedom, then the entire apparatus including the control system becomes complicated. Therefore, it is practically difficult to adopt the above construction.